Grain-oriented or non grain-oriented electrical steel has been used as a material for iron cores of most of medium to large sized transformers. In particular, as techniques having higher efficiency, as compared with existing techniques, and the necessity of research and development for the miniaturization and lightening of machines are required, research and development aimed at methods of producing high grade grain-oriented electrical steel is absolutely indispensable.
In particular, since grain-oriented electrical steel has to be easily magnetized and has to have high magnetic characteristics in a rolled direction thereof, a texture structure appearing when silicon (Si) is excessively added to ultra low carbon steel has to be artificially formed. However, such grain-oriented electrical steel may exhibit the characteristics of high grade grain-oriented electrical steel when a silicon (Si) component is contained in an amount of not less than 6.5% so as to enhance the magnetic properties thereof.
Also, the grain-oriented electrical steel is disadvantageous in that it has to be subject to a heat treatment at a high temperature and a nitrogen atmosphere so as to artificially form a texture structure known as a Goss structure. This is because the <100> crystal orientation that has a maximum magnetic induction value has to be controlled.
Meanwhile, although a method capable of improving the magnetic properties of electrical steel through control of the texture structure or a surface coating has been recently proposed, electrical steel used for transformers requires precision processing for suppressing tears, shearing, or bending of the electrical steel sheet that may be caused when the electrical steel strips are stacked. In the case in which an iron core is comparatively small, it is difficult to process the electrical steel strip, a volume of a portion of the core distorted by processing of steel to the overall volume of the core increases relatively, and thus the magnetic properties may be remarkably reduced.
To solve the above-mentioned limitations, a technique in which an electronic steel wire or an electrical steel wire is manufactured and then a wire rod for a small motor provided in a small transformer or a vehicle is manufactured has been developed. When electrical steel is manufactured in the form of a wire rod, a severe process control for rolling and surface defect suppression is not necessary and a yield drop due to lamination of electrical steel strips may be solved.
Japanese Unexamined Patent Application Publication No. 2001-115241 discloses a representative technique. The above-mentioned technique is to manufacture a material for electrical steel having excellent drawing workability, especially cold drawing workability as rolled, and discloses a component system containing Si in an amount range of 0.1-8% and C+N+O+S in a sum amount of not more than 0.015%. However, since the technique controls the content of carbon to an ultra low value, a Ruhrstahl-Heraues (RH) degassing process has to be added, and since composite deoxidization has to be performed with a relatively long vacuum degassing time, an increase in process costs is inevitable. Also, since chromium (Cr) has to be added up to a content range of 0.1-15% so as to enhance magnetic properties, a cost increase due to the addition of alloy elements is also inevitable.
One of techniques for compensating for the defects in the of the above patent process is that disclosed in Japanese Unexamined Patent Application Publication No. 2000-045051. The above patent discloses a silicon steel wire with a small deterioration in core loss and excellent workability in which the contents of carbon (C), nitrogen (N), oxygen (O), and sulfur (S) are controlled to, by weight, C+S+O+N<0.015%, the average grain size and the diameter of a wire rod after being drawn are controlled, and ≦2% Ni,≦2% Al, and ≦2% Cu are further added as alloy elements. However, the silicon steel wire disclosed in the above patent has drawbacks, such as an increase in manufacturing costs thereof due to an increase of additional contents of alloy elements, a lack of suggestions on magnetic properties through processes such as hot rolling, and a lack of clear suggestions on texture structure fraction.
Meanwhile, Japanese Unexamined Patent Application Publication No. 2001-131718 discloses steel wire in which the total content of C, S, O and N is suppressed to ≦0.025 wt. %, and the diameter of a drawn wire is controlled to 0.01 to 1.0 mm. However, the above patent also requires the essential addition of relatively highly priced alloy elements, for example, Cr, Ni, Cu, and the like, and has drawbacks, such as lack of suggestions for a concrete structure with regard to magnetic properties, and lack of suggestions for values of magnetic properties.
In particular, all of the above-mentioned patents have a common drawback, in that the magnetic properties of the silicon steel wires have values close to those of non grain-oriented electrical steels and a subsequent annealing treatment has to be performed so as to increase magnetic properties.